Abstract: "Sweet spot" identification and prediction is the key to develop tight gas reservoirs with low permeability.In order to explore the types and genesis of "sweet spots" in deep low permeability tight gas reservoirs with fewer wells in the sea,and to guide the exploration and development of oil and gas fields,the study investigated the "sweet spots" in X gas field in the Xihu Sag,East China Sea Shelf Basin,through multiple methods,such as core observation,cast thin section,cathodoluminescence and scanning electron microscopy thin section identification,grain size analysis,nuclear magnetic resonance,high pressure and constant pressure mercury intrusion porosimetry(MIP),porosity and permeability test and elemental analysis.First of all,the definition of "sweet spot" was explored,and three classes of "sweet spots" were identified based on the theoretical production capacity,physical property,pore structure and bound water content:"Sweet spot" Class Ⅰ is high yield gas reservoirs with Type Ⅰ pore structure being dominant and the irreducible water saturation mostly in the range of 22% to 57%; "Sweet spot" Class Ⅱ is moderate yield gas reservoirs with Type Ⅱ pore structure being dominant,followed by Type Ⅰ pore structure and Type Ⅲ pore structure being rare,and irreducible water saturation varies mostly between 45% and 55%; "Sweet spot" Class Ⅲ is low-yield gas reservoirs,with Type Ⅲ pore structure being dominant and a small number of IV pore structures developed,and irreducible water saturation mostly between 45% and 55%.It is believed that Class Ⅰ and Ⅱ "sweet spots" can be efficiently and economically developed at present,but Class Ⅲ "sweet spot" has "potentials" for future development.According to the research,we propose that (1) burial depth is the decisive factor in controlling the development of "sweet spot",thus no Class Ⅰ and Ⅱ "sweet spots" occur in areas with burial depth larger than 4 000 m in general; (2) sedimentary environment is the root cause controlling the development of "sweet spot"; (3) event sedimentation and diagenesis greatly impact the formation of Class Ⅰ and Ⅱ "sweet spot"; (4) dissolution is the main controlling factor for the development of Class Ⅲ "sweet spot"; (5) tectonic movement is a necessary condition for the formation and preservation of "sweet spots" in the study area.

Key words: “sweet spot” classification, origin of “sweet spot”, deep stratum, low permeability tight gas reservoir, Xihu Sag, East China Sea Shelf Basin